Wrong problem. Sun delivers about 900Wh per sq meter in a sunny day. That's 100x100 cm, or 10,000 sq cm. That gives about 90 mWh per sq cm. A phone that's 10 by 7 makes for 70 sq cm, or 6300 mWh, or 6 Wh.

N900 battery is 1300 mAh by 4V approx, so 5200 mWh, about 5 Wh roughly.

Math is nice but in real life you need: charge disspation that's around 20 percent, solar cell efficiency that's about 20 percent, 29 for the really cool stuff (remember, it works only with a limited spectrum).

So that's a 6-7 Wh battery from a panel that gives 6 W by 30%, or about 2Wh.

Ignoring the fact that no battery can sustain full charing current all the time (over 80 percent it's trickle), that still gives you 3 hour charge, on a sunny desert day, from 0 to about 80something percent.

Unless you are in the desert, tough.

It's not a matter of perfecting it. Solar cells have a theoretical eficiency of 29 percent. Sun delivers 1366 Wh per sq meter, but likely not on your latitude. So, basically, you need bigger sails.

And I haven't even tackled clouds, pollution, angle, heat disspation of electronics, variations of light flux, all of which kill figures and destroy batteries.